我真的很难弄清楚这个问题。当在 Fragments 与 RecyclerView 之间切换时，我得到了一个 NullPointerException 但只是以特定的顺序。代码和异常如下。先解释一下:

我有一个带有此选项卡式布局的 fragment ，如下所示:

这些选项卡中的 3 个使用包含 RecyclerView 的相同 fragment :Schedule、History 和 Test。如果我先选择它们，它们中的任何一个都可以工作。现在的问题是，当我转到我选择的第一个选项卡右侧的其他选项卡之一时!但是，如果我转到我选择的第一个的左侧，那将加载得很好。

如果我选择“测试”，然后转到“历史记录”，然后选择“安排”，一切都会正常进行

如果我选择 Test 然后 Schedule 这两个将起作用，但是然后转到 History 将导致 异常。

如果我选择 Schedule then History，则会发生异常。

如果我选择 History 然后 Schedule 这两个会起作用，但是然后去 Test 会发生异常

如果我选择 History then Test 异常将会发生

我扩展了 RecycleView 并中断了 onMeasure 以查看是否可以弄清楚发生了什么，我注意到 的 mAdapter RecyclerView 为 null。我在 RecyclerView 的 setAdapter() 内部进行了检查，它始终传递了一个有效的适配器。所以看起来 mAdapter 在某个时候被清除了。我只是不明白为什么它只会在我打开从左到右而不是从右到左的选项卡时发生...

任何帮助都将不胜感激，因为我对这个已经无能为力了!

这是 NullPointerException:

Link to Exception trace (不得不把它放在这里以避免字符限制)

编辑: 在捕捉到 NullPointerException 之后，也会弹出一个 IllegalStateException。看起来来自 LayoutManager。 Here is a link to that output

这是标签 View 的 fragment :

public class FKitViewer extends Fragment implements OnTabChangeListener {

    public static final String TAB_INFO = "Info";
    public static final String TAB_SCHEDULE = "Schedule";
    public static final String TAB_HISTORY = "History";
    public static final String TAB_TEST = "Test";
    public static final String TAB_REQS = "Reqs";

    private View mRoot;
    private TabHost mTabHost;
    private int mCurrentTab;

    @Override
    public void onAttach(Activity activity){
        super.onAttach(activity);
    }

    @Override
    public View onCreateView(LayoutInflater li, ViewGroup container,
                                Bundle savedInstanceState){
        mRoot = li.inflate(R.layout.kit_view, container);
        mTabHost = (TabHost) mRoot.findViewById(android.R.id.tabhost);
        setupTabs();
        return mRoot;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState){
        super.onActivityCreated(savedInstanceState);
        setRetainInstance(true);
        mTabHost.setOnTabChangedListener(this);
        mTabHost.setCurrentTab(mCurrentTab);
        updateTab(TAB_INFO, R.id.tab1);
    }

    private void setupTabs(){
        mTabHost.setup();
        mTabHost.addTab(newTab(TAB_INFO, "Info", R.id.tab1));
        mTabHost.addTab(newTab(TAB_SCHEDULE, "Schedule", R.id.tab2));
        mTabHost.addTab(newTab(TAB_HISTORY, "History", R.id.tab3));
        mTabHost.addTab(newTab(TAB_REQS, "Reqs", R.id.tab4));
        mTabHost.addTab(newTab(TAB_TEST, "Test", R.id.tab5));
    }

    private TabSpec newTab(String tag, String label, int tabContentId){
        TabSpec tabSpec = mTabHost.newTabSpec(tag);
        tabSpec.setIndicator(label);
        tabSpec.setContent(tabContentId);
        return tabSpec;
    }

    private void updateTab(String tabId, int placeholder){
        FragmentManager fm = getFragmentManager();
        if(fm.findFragmentByTag(tabId) == null){
            if(tabId.equals(TAB_INFO))
                fm.beginTransaction()
                    .replace(placeholder, new FListiesView(), tabId)
                    .commit();
            if(tabId.equals(TAB_SCHEDULE))
                fm.beginTransaction()
                    .replace(placeholder, new FKitSchedule(false), tabId)
                    .commit();
            if(tabId.equals(TAB_HISTORY))
                fm.beginTransaction()
                    .replace(placeholder, new FKitSchedule(true), tabId)
                    .commit();
            if(tabId.equals(TAB_REQS))
                fm.beginTransaction()
                    .replace(placeholder, new KitRequirements(), tabId)
                    .commit();
            if(tabId.equals(TAB_TEST))
                fm.beginTransaction()
                    .replace(placeholder, new FKitSchedule(false), tabId)
                    .commit();
        }
    }

    @Override
    public void onTabChanged(String tabId){
        Log.d("MNB", "Changing to tab: " + tabId);
        if(TAB_INFO.equals(tabId)){
            updateTab(tabId, R.id.tab1);
            mCurrentTab = 0;
            return;
        }
        if(TAB_SCHEDULE.equals(tabId)){
            getActivity().getIntent().getExtras().putBoolean("com.crummy.history", false);
            updateTab(tabId, R.id.tab2);
            mCurrentTab = 1;
            return;
        }
        if(TAB_HISTORY.equals(tabId)){
            getActivity().getIntent().getExtras().putBoolean("com.crummy.history", true);
            updateTab(tabId, R.id.tab3);
            mCurrentTab = 2;
            return;
        }
        if(TAB_REQS.equals(tabId)){
            updateTab(tabId, R.id.tab4);
            mCurrentTab = 3;
            return;
        }
        if(TAB_TEST.equals(tabId)){
            getActivity().getIntent().getExtras().putBoolean("com.crummy.history", true);
            updateTab(tabId, R.id.tab5);
            mCurrentTab = 4;
            return;
        }
    }
}

这是 fragment :

public class RVFKitSchedule extends Fragment 

实现 LoaderManager.LoaderCallbacks{

private String mGetURL = "/index.php/droid/kitreport/";
private String mHistoryURL = "/index.php/droid/kithistory/";
private String mUpdateStatusURL = "/index.php/order_status/update/";
private Boolean mIsHistory = false;

private LinearLayout mPbl;

private int mDialogStatusLevel = 0;
private int mDialogLineId = -1;

private List<ScheduleRowModel> mItems = null;
private FKVRecyclerView mRecyclerView;
private KitScheduleRVAdapter mRVAdapter;
private int mItemCount;
private RecyclerView.LayoutManager mLayoutManager;

private FragmentActivity mParentActivity;

public RVFKitSchedule(){

}

public RVFKitSchedule(boolean isHistory){
    mIsHistory = isHistory;
    if(mIsHistory)
        mGetURL = mHistoryURL;
}

@Override
public View onCreateView(LayoutInflater li, ViewGroup vg, Bundle b){
    super.onCreateView(li, vg, b);
    return li.inflate(R.layout.rv_main, vg, false);
}


@Override
public void onActivityCreated(Bundle icicle){
    super.onActivityCreated(icicle);

    final InputMethodManager imm = (InputMethodManager) mParentActivity.getSystemService(Context.INPUT_METHOD_SERVICE);
    imm.hideSoftInputFromWindow(getView().getWindowToken(), 0);
}

@Override
public void onViewCreated(View view, Bundle icicle){
    super.onViewCreated(view, icicle);
    mParentActivity = getActivity();
    mRecyclerView = (FKVRecyclerView)mParentActivity.findViewById(R.id.recyclerView);
    mLayoutManager = new LinearLayoutManager(mParentActivity);

    //mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    //mLayoutManager.
    mLayoutManager.scrollToPosition(0);
    mRecyclerView.setLayoutManager(mLayoutManager);

    RecyclerView.ItemDecoration itemDecoration = 
            new DividerItemDecoration(mParentActivity, DividerItemDecoration.VERTICAL_LIST);
    mRecyclerView.addItemDecoration(itemDecoration);
    mItems = new ArrayList<ScheduleRowModel>();
    mRVAdapter = new KitScheduleRVAdapter(mItems, mIsHistory);
    //mRecyclerView.setAdapter(null);
    if(mRecyclerView.getAdapter() != null)
        mRecyclerView.swapAdapter(mRVAdapter, true);
    else
        mRecyclerView.setAdapter(mRVAdapter);

    mRecyclerView.addOnItemTouchListener(
            new RecyclerItemClickListener(mParentActivity, new RecyclerItemClickListener.OnItemClickListener(){
                @Override
                public void onItemClick(View v, int position){
                    String status = ((TextView)v.findViewById(R.id.sr_status)).getText().toString();
                    if(status.equals("Open"))
                        mDialogStatusLevel = 0;
                    else if(status.equals("In Production"))
                        mDialogStatusLevel = 1;
                    else if(status.equals("Waiting"))
                        mDialogStatusLevel = 2;
                    else if(status.equals("Ready"))
                        mDialogStatusLevel = 3;
                    else if(status.equals("Shipped"))
                        mDialogStatusLevel = 4;
                    int lineId = Integer.parseInt(((TextView)v.findViewById(R.id.sr_order_line_id)).getText().toString());
                    mDialogLineId = lineId;
                    OrderStatusDialog df = new OrderStatusDialog(mParentActivity, mDialogStatusLevel){
                        @Override
                        protected void onDialogClick(DialogInterface di, final int which){
                            Runnable update = new Runnable(){
                                @Override
                                public void run(){
                                    updateStatus(mDialogLineId, which);
                                }
                            };
                            Thread t = new Thread(null, update, "MagentoBackground");
                            t.start();
                            try{
                                t.join();
                            }catch(InterruptedException e){
                                e.printStackTrace();
                            }
                            restartLoading();
                            dismiss();
                        }
                    };
                    df.show(getFragmentManager(), "dialog");
                }
            }){
                @Override
                public void onItemLongClick(View v, int pos){
                    String s = ((TextView)v.findViewById(R.id.sr_pn)).getText().toString();
                    String n = ((TextView)v.findViewById(R.id.sr_pn)).getText().toString();

                    CheckItemTypeTask task = new CheckItemTypeTask(s, null);
                    task.execute(mParentActivity);
                    int type = -1;
                    try{
                        type = task.get();
                    }catch(Exception e){
                        e.printStackTrace();
                    }
                    if(type == ItemType.PART){
                        openPartViewer(s);
                    }else if(type == ItemType.KIT){
                        openKitViewer(s,n);
                    }else if(type == ItemType.ERROR){
                        Toast.makeText(mParentActivity, "Error getting item type!", Toast.LENGTH_LONG);
                    }else{
                        Toast.makeText(mParentActivity, "Invalid item type!", Toast.LENGTH_LONG).show();
                    }
                }
            }
    );

    if(mParentActivity.getIntent().getExtras() != null){
        Bundle b = mParentActivity.getIntent().getExtras();
        if(!b.isEmpty()){
            mGetURL += b.getString("com.crummy.kitNum");
        }
    }

    mPbl = (LinearLayout)mParentActivity.findViewById(R.id.main_pbl);
    mPbl.setVisibility(View.VISIBLE);


    LoaderManager lm = getLoaderManager();
    if(lm.getLoader(0) != null){
        lm.initLoader(0, null, this);
    }
    startLoading();
}

protected void startLoading(){
    getLoaderManager().initLoader(0, null, this);
}

protected void restartLoading(){
    getLoaderManager().restartLoader(0, null, this);
}

@Override
public Loader<Void> onCreateLoader(int arg0, Bundle arg1){
    Log.d("MNB", "FKitSchedule: onCreateLoader");
    AsyncTaskLoader<Void> loader = new AsyncTaskLoader<Void>(mParentActivity){
        @Override
        public Void loadInBackground(){
            try{
                getLines();
            }catch(Exception e){
                e.printStackTrace();
            }
            return null;
        }
    };
    loader.forceLoad();
    return loader;
}

@Override
public void onLoadFinished(Loader<Void> arg0, Void arg1){
    Log.d("MNB", "FKitSchedule: onLoadFinished");
    mPbl.setVisibility(View.GONE);
}

@Override
public void onLoaderReset(Loader<Void> arg0){

}

protected boolean openKitViewer(String kitNum, String kitName){
    boolean result = false;
    Intent i = new Intent(mParentActivity, GenFragmentActivity.class);
    i.putExtra("com.crummy.frag_layout_id", R.layout.kit_view_frag);
    i.putExtra("com.crummy.kitNum", kitNum);
    i.putExtra("com.crummy.kitName", kitName);
    startActivity(i);
    return result;
}

protected boolean openPartViewer(String itemNum){
    boolean result = false;
    Intent i = new Intent(mParentActivity, PartInfoFragmentActivity.class);
    i.putExtra("com.crummy.partnum", itemNum);
    startActivity(i);
    return result;
}

protected void getLines(){
    Log.d("MNB", "FKitSchedule: getLines() ");
    try{
        HttpEntity response = HttpHelper.tryHttp(mParentActivity, mGetURL, null);
        if(response == null)
            return;
        String res = EntityUtils.toString(response);
        if(res.equals("[]")){
            mParentActivity.runOnUiThread(noKitsError);
            return;
        }
        JSONArray jsona = new JSONArray(res);
        mItems = new ArrayList<ScheduleRowModel>();
        Log.d("MNB", "jsona.length() = " + jsona.length());
        for(int i=0; i < jsona.length(); i++){
            Log.d("MNB", "Reading json line " + i);
            JSONObject j = (JSONObject) jsona.get(i);
            ScheduleRowModel srm = new ScheduleRowModel();
            srm.custPo = j.getString("custPo");
            srm.dueDate = j.getString("dueDate");
            srm.partNum = j.getString("KitNumber");
            srm.qty = j.getInt("quantity");
            srm.status = j.getString("status");
            srm.lineId = j.getInt("id");
            srm.company = j.getString("name");
            srm.rev = j.getString("rev");
            if(!mIsHistory){
                if(!j.getString("note").equals("null"))
                    srm.note = j.getString("note");
            }
            mItems.add(srm);
        }
    }catch(Exception e){
        e.printStackTrace();
    }
    mParentActivity.runOnUiThread(returnRes);
}

private void addItemToList(ScheduleRowModel model){
    mItemCount++;       
    mRVAdapter.addData(model);
}

private HttpEntity updateStatus(int lineId, int statusId){
    HttpEntity resEntityPost = null;
    try{
        HttpClient client = new DefaultHttpClient();
        String host = HttpHelper.getHost(mParentActivity);
        HttpPost post = new HttpPost(host + mUpdateStatusURL);
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
        nameValuePairs.add(new BasicNameValuePair("line_id", Integer.toString(lineId)));
        nameValuePairs.add(new BasicNameValuePair("status", Integer.toString(statusId)));
        post.setEntity(new UrlEncodedFormEntity(nameValuePairs));
        HttpResponse res = client.execute(post);
        resEntityPost = res.getEntity();
    }catch(Exception e){
        e.printStackTrace();
    }
    return resEntityPost;
}
protected Runnable returnRes = new Runnable(){
    @Override
    public void run(){
        Log.d("MNB", "FKitSchedule returnRes ");
        mItemCount = 0;
        mRVAdapter.clear();
        if(mItems != null && mItems.size() > 0){
            for(int i = 0; i < mItems.size(); i++){
                addItemToList(mItems.get(i));
            }
        }
    }
};

private Runnable noKitsError = new Runnable()
{
    public void run(){
        Toast t = Toast.makeText(mParentActivity, "No orders for this kit.", Toast.LENGTH_SHORT);
        t.show();
    }
};

这是 RecyclerView.Adapter:

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.Locale;

import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class KitScheduleRVAdapter extends RecyclerView.Adapter<KitScheduleRVAdapter.ScheduleRowViewHolder>{

    private List<ScheduleRowModel> items;
    private boolean mIsHistory = false;

    KitScheduleRVAdapter(List<ScheduleRowModel> modelData, boolean isHistory){
        Log.d("MNB", "Constructing KitScheduleRVAdapter");
        if(modelData == null){
            throw new IllegalArgumentException(
                    "modelData must not be null");
        }
        this.items = modelData;
        mIsHistory = isHistory;
    }

    @Override
    public ScheduleRowViewHolder onCreateViewHolder(ViewGroup vg, int viewType){
        Log.d("MNB", "KitScheduleRVAdapter.onCreateViewHolder()");
        View itemView = LayoutInflater
                        .from(vg.getContext())
                        .inflate(R.layout.schedule_row, vg, false);
        return new ScheduleRowViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(ScheduleRowViewHolder vh, int pos){
        Log.d("MNB", "Binding Viewholder at pos "+pos);
        ScheduleRowModel model = items.get(pos);
        vh.date.setText(model.dueDate);
        vh.po.setText(model.custPo);
        vh.partnum.setText(model.partNum);
        vh.qty.setText(""+model.qty);
        vh.status.setText(model.status);
        vh.id.setText(""+model.lineId);
        vh.company.setText(model.company);
        vh.rev.setText(model.rev);
        vh.note.setText(model.note);

        Calendar cutoff = Calendar.getInstance();
        cutoff.add(Calendar.DAY_OF_MONTH, 14);
        Date co = cutoff.getTime();

        SimpleDateFormat s = new SimpleDateFormat("yyyy-MM-dd", Locale.US);
        Date d = new Date();
        try {
            d = s.parse(model.dueDate);
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        View statusBar = vh.statusBar;
        if(!mIsHistory){
            if(model.status.equals("Ready")){
                statusBar.setBackgroundResource(R.color.sh_ready);
            }else if(model.status.equals("Waiting")){
                statusBar.setBackgroundResource(R.color.sh_waiting);
            }else if(model.status.equals("In Production")){
                statusBar.setBackgroundResource(R.color.sh_inprod);
            }
            else if(d.before(co)){
                statusBar.setBackgroundResource(R.color.sh_soon);
            }else{
                statusBar.setBackgroundResource(R.color.sh_open);
            }
        }
        else
        {
            statusBar.setBackgroundResource(R.color.sh_ready);
        }
    }

    @Override
    public int getItemCount(){
        return items.size();
    }

    public void addData(ScheduleRowModel data){
        items.add(data);
        this.notifyItemInserted(items.size() - 1);
    }

    public void clear(){
        Log.d("MNB", "KitScheduleRVAdapter.clear()");
        int itemsCleared = items.size();
        items.clear();
        notifyItemRangeRemoved(0, itemsCleared);
    }

    public final static class ScheduleRowViewHolder extends RecyclerView.ViewHolder{
        TextView date;
        TextView po;
        TextView qty;
        TextView partnum;
        TextView status;
        TextView id;
        TextView company;
        TextView rev;
        TextView note;
        View statusBar;

        public ScheduleRowViewHolder(View itemView){
            super(itemView);
            date = (TextView) itemView.findViewById(R.id.sr_date);
            po = (TextView) itemView.findViewById(R.id.sr_po);
            qty = (TextView) itemView.findViewById(R.id.sr_qty);
            partnum = (TextView) itemView.findViewById(R.id.sr_pn);
            status = (TextView) itemView.findViewById(R.id.sr_status);
            id = (TextView) itemView.findViewById(R.id.sr_order_line_id);
            company = (TextView) itemView.findViewById(R.id.sr_company);
            rev = (TextView) itemView.findViewById(R.id.sr_rev);
            note = (TextView) itemView.findViewById(R.id.sr_note);
            statusBar = itemView.findViewById(R.id.sr_status_bar);
        }
    }
}

最佳答案

您应该在 onViewCreated() 而不是 onActivityCreated() 中执行 View 分配(和 LayoutManager 分配)——onActivityCreated() 不是当您的 Fragment 通过 ViewPager 分离并重新附加时，不会再次发生。 View 在消失和返回时将被销毁 (onDestroyView()) 并重新创建 (onCreateView())，但 RecyclerView 永远不会在这种情况下获取它的 LayoutManager。

编辑:从评论中提取，另一个问题是使用父 Activity 来解析 RecyclerView 而不是使用 Fragment 的 查看。由于 findViewById() 执行深度优先搜索，如果您附加了多个包含具有相同 ID 的 View 的 Fragments，您最终可能会得到错误的 View ，但一切仍然会编译和运行，只是会产生意想不到的结果。使用在 onViewCreated() 中返回的 view 将您的 View 搜索限制为只是您在 onCreateView() 中膨胀的布局>.

关于java - 在包含 RecyclerView 的 fragment 之间切换时出现 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26790102/